Q.: We completed a floor pour and were surprised to find fine pattern cracks in the surface the next day. The mix wasn't particularly rich, with a cement content of 550 pounds per cubic yard. We carefully limited slump to 3 inches and didn't overtrowel the surface. Because the weather was hot and dry, we applied a curing compound as soon as the bleedwater sheen had left the surface. However, the cracks were quite visible the next day and the owner was upset. We made six pours on the job and had this problem with only one of them. What did we do wrong?
A.: There's a possibility that the cracks were only in the curing membrane and not in the concrete. The American Concrete Institute's "Standard Practice for Curing Concrete" (ACI 308-81) warns that map cracking of a membrane film can be caused by applying curing compound to a dry surface before bleeding has stopped. This is most likely to happen when the evaporation rate exceeds 0.2 pound of water per square foot per hour. When this happens, water evaporates faster than it can rise to the surface through bleeding. If a membrane-forming curing compound is applied to a dry-appearing surface, evaporation will be temporarily stopped but bleeding may continue. This floats the membrane film which cracks when the water later evaporates.
To find out if cracks are in the membrane instead of the concrete surface, you might try rubbing a small area with additional liquid curing compound. If the cracks disappear you'll know they were in the membrane. Then apply more curing compound over the cracked area to ensure that internal moisture is retained.